Conventional horizontal landing of a fixed-wing air vehicle (having a forward speed sufficient to sustain aerodynamic flight), particularly where the forward thrust vector is fixed with respect to the air vehicle, is conventionally accompanied by an increase in lift and reduction in air speed. Where the air vehicle is a powered air vehicle, landing is also conventionally accompanied by a reduction in engine power output, and the engine is often at idle during the landing maneuver until at least touchdown, to reduce airspeed and to provide a reasonable glide angle. While in some air vehicles, the engine power can be significantly increased after touchdown to provide reverse thrust to further reduce the forward speed of the air vehicle, maintaining a forward thrust after idle setting conventionally adversely affects landing performance.
In addition, conventional horizontal landings are subject to the conventional speed safety constraint, in which the airspeed of the air vehicle is conventionally maintained at least at 20% higher than the respective stall speed, as long as the air vehicle is airborne. Furthermore, conventional horizontal landings are often accompanied with the deployment of high lift devices and the use of drag inducing devices to provide increased glide angles.
Some types of air vehicles have wings which exhibit mild-stall characteristics at high lift, and are at least partly based on a two-element aerofoil design. In this connection, two-element slotted aerofoils are well known, and are disclosed for example in U.S. Pat. Nos. 8,109,473 and 7,992,827, assigned to the present assignee.
Slotted aerofoils are two-element aerofoils composed of a first element (the main aerofoil body), and a second element (for example in the form of a flap or aileron). The second element is separated from the first element by a slot which is open for the airflow at any deflection of the second element. In one class of such slotted aerofoils, the slot is permanent, and facilitates actuation of the second element through positive or negative deflection angles. Such slotted aerofoils can be designed for cruising/loitering flight at high lift coefficients, and rely on a second element rotation around an external fixed hinge point for adjustment of the aerofoil to different flight regimes. There are also cases in which the second element is rigidly connected with respect to the first element, providing a mechanically simple system with high maximum lift characteristics.
In general, stall characteristics of such aerofoils tend to deteriorate as maximum lift is increased, resulting in more difficulties when attempting to comply with considerations of flight safety (complying with the speed safety constraint) and to avoid unfavorable stall patterns. This is especially relevant for high-lift, long endurance wings of some UAV's, such as for example the Heron high-lift long endurance UAV, manufactured by IAI, Israel. In the Heron UAV, optimum endurance performance is achieved via high loitering lift coefficients, which requires high maximum lift.
However, in some cases, such slotted aerofoils are configured to provide mild-stall characteristics at high lift, resulting in a smooth plateau of lift coefficients at post-stall angles of attack. Specifically, mild stall is characterized by almost constant level of the lift at post-stall domain and is associated with slowly creeping trailing edge separation that moderates the rate of lift losses at high angles of attack, typically resulting in an approximately constant lift coefficient (up to about 5% of the maximum lift coefficient for at least about 7° after the stall angle of attack, or within between about 5% and about 10% of the maximum lift coefficient for at least about 5° after the stall angle of attack).
The stall angle of attack can be defined as the angle of attack at which maximum lift coefficient (or up to about 95% or 96% or 97% or 98% or 99% of maximum lift coefficient) is first realized. The stalling speed is dependent on the weight (W) of the air vehicle, maximum lift coefficient (CL max), wing area (Sw), and air density (ρ), and is generally defined as:Vstall=(2W/(ρ*CL max*Sw))0.5 
For example, mild-stall characteristics at an extended range of post-angle angles of attack can be achieved relying on the concept of the so-called mild stall ramp (MS-ramp) at the aft portion of the upper surface of the main body (see for example U.S. Pat. No. 8,109,473 (ref 1 above), Nagel et al (Ref 2 above: “Development of High-Lift UAV Wings”, 24th AIAA Applied Aerodynamics Conference, San Francisco, Calif., 5-8 Jun. 2006), and Shepshelovich (Ref 3 above: “The Progress in Development of UAV Wings”, International Conference ICAUV-2009, Bangalore, India, 2009)).
Such conventional two-element aerofoils also provide a longitudinal overlap (i.e., generally defined as an overlap in a direction generally parallel to the reference line of the aerofoil), between the trailing edge of the primary element (also referred to interchangeably herein as the trailing end of the primary element) and the leading edge of the secondary element (also referred to interchangeably herein as the leading end of the secondary element). The longitudinal overlap is positive when part of the primary element is superposed over part of the secondary element (i.e., when viewed in a direction normal to the reference line of the aerofoil, for example), and the positive direction is in a direction towards the leading edge of the aerofoil. Conversely, the longitudinal overlap is negative when there is no such superposition, and the trailing end of the primary element is longitudinally spaced away from the leading end of the secondary element (i.e., when viewed in a direction normal to the reference line of the aerofoil, for example), and the negative direction is in the direction towards the trailing edge of the aerofoil. The aforesaid “longitudinal overlap” is also referred to herein interchangeably as “overlap” or “axial overlap”.
Similarly, a gap can also be defined for the two-element aerofoils, as the “vertical” spacing between the trailing end of the primary element and the secondary element, i.e., in a direction generally orthogonal to the direction of the longitudinal overlap, i.e., in a direction generally orthogonal to the reference line of the aerofoil. Alternatively, the gap can be defined in a direction parallel to the thickness of the aerofoil.
Referring to FIG. 1, Abbott et al (Ref 7 above: “Theory of Wing Sections”, Dover Publication, book, 1959”) provides a contour map of maximum lift coefficient as a function of slot overlap and gap. The contours show that maximum lift coefficient for the NACA 23012 wing section was obtained for a longitudinal overlap of about 1% and gap of about 1.2%. As the longitudinal overlap is increased positively (i.e., in the positive direction, towards the aerofoil leading edge), for example to +2%, the maximum lift coefficient drops to about 2.75, and as the longitudinal overlap is increased negatively (i.e., in the negative direction, towards the aerofoil trailing edge), for example to 0%, the maximum lift coefficient again drops to about 2.75. Similar effects are observed as the gap is increased or decreased from about just about 1.2%. It is to be noted that the particular form of the contour map is aerofoil-dependent as well as Reynolds number-dependent.
Thus, conventionally, two-element slotted aerofoil design seeks to optimize performance by providing a positive longitudinal overlap close to zero, typically between 0° to 3° for example, for the range of flap angles δ in which maximum lift is to be maximized. For example the range of flap angle δ can be between about 25° and 30°, often required for take-off and landing performance, for example, and generally in which the flow over the secondary element is fully attached. Accordingly it follows that, if the second element of such two-element slotted aerofoils is further deflected to flap angles δ associated with use as airbrake (wherein generally, the flow over the secondary element is fully detached to maximize drag, for example flap angle δ between about 60° and 80°) about a fixed hinge point, the resulting overlap becomes negative, as illustrated in FIGS. 2(a) and 2(b), for example. Concurrently, the lift coefficient drops rapidly as flap angle δ increases past about 30° and continues dropping steadily past 60° (FIG. 3).
Indeed, while conventional airbrakes for air vehicles are designed to provide high levels of drag via large deflections of the airbrakes, such high levels of drag are conventionally coupled with a significant drop in maximum lift characteristics at these high deflections. Thus, while maximum lift of high-lift devices is realized with fully attached flow on the deflected flap segments (i.e., at conventional positive flap deflections), such flap segments when used as airbrakes (i.e., at airbrake flap deflections) rely on fully separated flow on the flap segment for producing maximum possible drag for deployed airbrake (δairbrake˜+60° to +80°).
Such a conventional airbrake concept is described for example in Steinbuch et al (Ref 4 above: “Development of UAV Wings—Subsonic Designs”, 41st Aerospace Sciences Meeting, Reno, Nev., 6-9 Jan. 2003), and was adopted in the development of the aforementioned IAI Heron UAV.
Providing a positive longitudinal overlap to maximize the maximum lift coefficient at flap angles associated with maximum lift conditions has become the conventional principle for design of high-lift devices (see, for example, the cases presented in McCormick (Ref. 5 above: “Aerodynamics Aeronautics and Flight Mechanics”, John Wiley & Sons, book, 2nd edition, 1995). Such high-lift devices may be of single-slotted flap type, for example. Conventionally, the optimum lift performance of these devices is achieved for positive overlap, or overlap close to zero (between −1.0 and 3.0% of chord) at flap deflections associated with maximum lift and almost fully attached flow on the high lift devices, and at flap deflections not greater than about 30°.
It is to be noted that comparatively mechanically simple designs for high-lift devices, for example as provided on at least some IAI high-lift, two-element wings, rely on single-slotted flap with a fixed hinge point location. In such cases, the maximum positive conventional flap deflections with fully attached flow are around δ˜+20° to +30°, depending on the flight Reynolds numbers (for example Reynolds numbers between about 0.2*106 to about 2.0*106), providing for maximizing the maximum lift coefficient. In such cases, flap deflections above δ˜+30° require a simple continuation of the flap rotation around the fixed hinge point, when the flap segment becomes fully separated and begins to operate in airbrake mode. In such cases, this continuation of the flap rotation around the fixed hinge point to large deflections angles above δ˜30°, and including 60° to 80°, or to more than 80°, automatically results in a longitudinal overlap that rapidly increases negatively, for example −3.5% of the chord at δ˜60°.